Beam forming method

ABSTRACT

In a radio communication system, a beam is formed between subscriber stations and a base station which has an antenna device with several antenna elements. The antenna elements emit a downlink signal based on respective weighing with coefficients of a weighting vector. A plurality of weighting vectors are determined at the subscriber station in an initialization phase and transmitted to the base station. In a subsequent working phase, the subscriber station selects a dominant weighting vector from the weighting vectors and transmits a designation of the selected weighting vector to the base station.

[0001] The invention relates to a method for beamforming in a radiocommunications system having a base station whose associated antennadevice has a number of antenna elements, so that spatial resolution ispossible in the beamforming.

[0002] In radio communications systems, messages (speech, pictureinformation or other data) are transmitted via transmission channels bymeans of electromagnetic waves (radio interface). The transmission takesplace both in the downlink direction from the base station to thesubscriber station and in the uplink direction from the subscriberstation to the base station.

[0003] Signals which are transmitted using electromagnetic waves aresubject, inter alia, to disturbances due to interference during theirpropagation in a propagation medium. Disturbances caused by noise may becaused, inter alia, by noise in the input state of the receiver. Due todiffraction and reflections, signal components pass over differentpropagation paths. Firstly, this means that a signal can arrive at thereceiver more than once, in each case from different directions, withdifferent delays, attenuations and phase angles, and, secondly,components of the received signal may be superimposed coherently withchanging phase relationships in the receiver, leading to cancelationeffects on a short-term timescale (fast fading) there.

[0004] DE 197 12 549 A1 discloses the use of intelligent antennas (smartantennas), that is to antenna arrangements having a number of antennaelements, in order to increase the transmission capacity in the uplinkdirection. These allow the antenna gain to be deliberately aligned in adirection from which the uplink signal is coming.

[0005] Various methods for spatial signal separation for the uplink anddownlink directions are known from A. J. Paulraj, C. B. Papadias,“Space-time processing for wireless communications”, IEEE SignalProcessing Magazine, November 1997, pages 49-83.

[0006] Particular difficulties occur in the downlink direction, that isto say from the base station to the subscriber station, since thebeamforming has to be carried out before the transmitted signals areinfluenced by the radio channel. An algorithm for beamforming in thedownlink direction is known from R, Schmalenberger, J. J. Blanz, “Acomparison of two different algorithms for multi antenna C/I balancing”,Proc. 2nd European Personal Mobile Communications Conference (EPMCC),Bonn, Germany, September 1997, pages 483-490, which is based on a directpropagation path (visual link) between the base stations and thesubscriber stations and iterative calculation of beamforming vectors.The entire, complicated, iterative calculation must be repeated wheneverthe characteristics of the transmission channel change.

[0007] DE 198 03 188 A discloses a method in which a spatial covariancematrix is defined for a link from a base station to a subscriberstation. An eigen vector is calculated in the base station from thecovariance matrix, and is used as a beamforming vector for that link.The transmission signals for the link are weighted with the beamformingvector and are supplied to antenna elements for emission. Intracellinterference is not included in the beamforming process owing to the useof joint detection, for example in the terminals, and any corruption ofthe received signals by intercell interference is negligible.

[0008] In an environment with multipath propagation, this method clearlydetermines a propagation path with good transmission characteristics andconcentrates the transmission power of the base station physically onthis propagation path. However, using this approach, it is impossible toprevent the possibility of interference on this transmission pathleading to signal cancelation, and hence to interruptions in thetransmission, in the short term.

[0009] The recommendations from the 3GPP (3rd Generation PartnershipProject, http://ww.3gpp.org) therefore provide methods in which thesubscriber station estimates a short-term channel impulse response h_(m)for the channel from the m-th antenna element to the subscriber station,and calculates weighting factors w_(m) which intended to be used forweighting the transmission signal before it is transmitted by the m-thantenna element. Corresponding concepts are also dealt with in M.Raitola, A. Hottinen and R. Wichmann, “Transmission diversity inwideband CDMA”, which appeared in the Proceedings of the 49th IEEEVehicular Technology Conf. Spring (VTC 99 Spring), pages 1545-1549,Houston, Tex. 1999.

[0010] One serious problem with this procedure is that the vector of theweighting factors which is estimated by the subscriber station must betransmitted to the base station and that, in accordance with theRecommendations from the 3GPP, only a narrow bandwidth of 1 bit per timeslot (slot) is available for this purpose. The vectors can thus betransmitted in only a coarsely quantized form. If the channel changesquickly and the weights need to be updated from one time slot to thenext, only two different relative phase angles of the antenna elementscan be set. If the channel is changing relatively slowly and, forexample, four time slots are available for transmission of the vector,16 different values of the vector can then be represented.

[0011] However, the known concepts reach their limits when the number ofantenna elements at the base station is greater than 2, since thebandwidth which is required for transmission of the vector increaseswith its number of components, that is to say with the number of antennaelements. This means that, although a large number of antenna elementswould be desirable on the one hand in order to make it possible to alignthe transmission beam as accurately as possible, the limited availablebandwidth on the other hand means that the weighting vector cannot beupdated sufficiently often as would be necessary for matching to fastfading.

[0012] The invention is based on the object of specifying an improvedmethod for beamforming which allows more reliable forming of thedownlink beam.

[0013] This object is achieved by the method according to the inventionhaving the features of patent claim 1. Developments of the invention canbe found in the dependent claims.

[0014] The method according to the invention for data transmission isused in a radio communications system having a base station andsubscriber stations. The subscriber stations are, for example, mobilestations, that is to say in a mobile radio network, or fixed stations,that is to say in so-called subscriber access networks for wire-freesubscriber access. The base station has an antenna device (smartantenna) with a number of antenna elements. The antenna elements allowdirectional reception and directional transmission of data via the radiointerface.

[0015] The method according to the invention distinguishes between aninitialization phase, which is in each case carried out at relativelylong time intervals corresponding to a large number of time slots forthe relevant subscriber station, and a working phase, whose steps arecarried out more frequently, for example up to once per time slot. Inthe initialization phase, a number of so-called first weighting vectorsare determined, which are used in a subsequent working phase in theradio communications system in order to define a current weightingvector, which is actually used for beamforming, as new in each case, foreach cycle of the working phase. The processing complexity associatedwith the determination of the weighting vectors thus occurs onlyrelatively rarely, in the initialization phases; the definition of thecurrent weighting vector, which, for example, requires only a choice orthe formation of a linear combination of the first weighting vectors,can in contrast be carried out as frequently as necessary in order tocompensate for transmission interruptions caused by fast fading.

[0016] A first preferred refinement of the method provides for the firstweighting vectors to be determined on the basis of measurements of thedownlink transmission. This procedure is particularly expedient forradio communications systems which use different frequencies for theuplink and downlink since, in radio communications systems such asthese, fast signal fading at the different frequencies is notcorrelated. Furthermore, steps in the method according to the invention,which are carried out both for determining the first weighting vectorsin the initialization phase and for redefinition of the currentweighting vectors in the working phase, therefore need be carried outonly at the subscriber station. This avoids the processing complexitybeing duplicated, and circuit components for carrying out the methodsteps need also be provided only once, at the subscriber station.

[0017] In this case, the first weighting vectors determined at thesubscriber station are expediently transmitted to the base station inthe initialization phase, and the redefinition of the current weightingvector is carried out in the working phase, by the subscriber stationselecting a dominant weighting vector from the determined firstweighting vectors and transmitting a designation of the selecteddominant weighting vector to the base station. Since this transmissionneed not take place in each individual time slot of the subscriberstation, a dedicated channel can be allocated to it at times, or thetransmission of user data such as speech from the subscriber station tothe base station can be interrupted or constrained in individual timeslots, in order to create transmission bandwidth for the transmission ofthe weighting vectors. These weighting vectors can thus be transmittedwith considerably better resolution than is possible with theconventional methods, with the transmission bandwidth of 1 bit per timeslot.

[0018] The weighting vectors in each case correspond to emissiondirections of the antenna device of the base station. Fast fading canadmittedly lead to short-term adverse effects on the transmission onsuch a directional propagation path; the directions themselves in whichthe downlink signal must be emitted in order to reach the subscriberstation will change only slowly, however, even if the subscriber stationis moving, for example over a time scale of seconds to minutes. Theweighting vectors which are transmitted to the base station can thus beused over a time period of appropriate length for the beamforming, evenif the weighting vectors at any given time do not all allow high-qualitytransmission. If the transmission quality of a weighting vector which isused at a given time deteriorates, the base station must change at shortnotice to a different weighting vector which allows satisfactorytransmission, or the best possible transmission. This weighting vectoris in this case referred to as a dominant weighting vector. Since theindividual coefficients of this weighting vector are already known atthe base station, they no longer need be transmitted individually in theworking phase, and it is sufficient to transmit only one designationwhich allows the base station to select the dominant weighting vectordesired by the subscriber station from those stored in it, and to usethis for transmission. The amount of information which is required fortransmitting such a designation is completely independent of theresolution with which the coefficients of the weighting vectors havebeen transmitted in the initialization phase, and it is also independentof the number of coefficients of each vector, that is to say it isindependent of the number of antenna elements in the antenna device atthe base station. This amount of information increases onlylogarithmically with the number of weighting vectors which aretransmitted to the base station. This means that high-precisionbeamforming is possible in the working phase of the subscriber station,with a minimal bandwidth requirement for transmission of thedesignation.

[0019] A first spatial covariance matrix of the received downlink signalis preferably produced in the initialization phase, and eigen vectors ofthis first covariance matrix are determined, and are transmitted asweighting vectors to the base station.

[0020] This first covariance matrix can be produced as standard for theentire downlink signal received by the subscriber station. Since theindividual components of the downlink signal received by the subscriberstation differ not only in the path that they have traveled but also inthe delay time required for this path, it is more appropriate for thefirst covariance matrix to be produced individually for each tap of thedownlink signal.

[0021] Those eigen vectors of the totality of eigen vectors of the firstcovariance matrix or matrices which have the highest eigen values arepreferably determined, since these correspond to the propagation pathswith the least attenuation.

[0022] In order to obtain a representative conclusion about the qualityof the individual transmission paths, it is also expedient for eachfirst covariance matrix to be averaged over a large number of time slotsin the downlink signal.

[0023] In order to determine that weighting vector which is the mostsuitable at any given time in the operating phase, a second spatialcovariance matrix is preferably produced, and the eigen vector that ischosen from the determined eigen vectors as being the dominant weightingvector is that which has the highest eigen value with the secondcovariance matrix. This second spatial covariance matrix may be producedfrom new, for example for each time slot allocated to the subscriberstation.

[0024] In order to make it possible to distinguish between thecomponents from the individual antenna elements when producing thecovariance matrices, it is expedient for each antenna element toperiodically emit a training sequence, which is known to the subscriberstation and is orthogonal with respect to the training sequences of theother antenna elements, and for the weighting vectors to be determinedon the basis of the training sequences received by the subscriberstation.

[0025] One specific refinement allows the number of determined weightingvectors to be two; in this case, one bit is sufficient to identify therespectively dominant weighting vector in the working phase, and thisbit can be transmitted in each time slot allocated to the subscriberstation.

[0026] A greater number of weighting vectors can also be determined,preferably a power of 2^(n), in which case n bits are required toidentify the dominant weighting vector. The transmission of thisdesignation can be distributed over a number of time slots; if a bitsare available for transmission in each time slot, n/a time slots arerequired, and the weighting vector which is specified by the designationis inserted into the n/a time slots which immediately follow thecomplete transmission of the designation.

[0027] A second preferred refinement allows the first weighting vectorsto be determined on the basis of measurements of the uplinktransmission. This procedure has the advantage that there is no need totransmit the coefficients of the first weighting vectors from thesubscriber station to the base station. A method such as this istherefore more compatible with existing mobile radio systems, which donot provide such transmission.

[0028] Although the fast fading in mobile radio systems which usedifferent frequencies for the uplink and downlink is different for thetwo transmission directions, this does not, however, have any disturbinginfluence on the determination of the first weighting vectors, if thelatter are obtained by time averaging, in particular on the basis of anaveraged covariance matrix.

[0029] In this case as well, it is preferable for the first weightingvectors in each case to be eigen values of a covariance matrix, sincethese eigen values in each case correspond to an individual propagationpath for the radio signal which is interchanged at the same time,possibly on a number of different paths, between the base station andthe subscriber station. If a direct propagation path (LOS, line ofsight) exists between the subscriber station and the base station, whichthe base station can determine from the reception statistics of theuplink signal, then it is sufficient for it to transmit the downlinksignal weighted with a single weighting vector corresponding to thistransmission path. In this way, the transmission power of the basestation is specifically directed at the direct transmission path, whileother transmission paths of poorer quality are not deliberately suppliedwith transmission power.

[0030] If there is no direct transmission path, a linear combination offirst weighting vectors may be used as the current weighting vector.This corresponds to deliberately sharing of the transmission power ofthe base station between a limited number of transmission pathscorresponding to the number of current weighting vectors included in thelinear combination. If, in a situation such as this, one of thetransmission paths fails at short notice due to fast fading, there is ahigh probability that at least one other weighting vector in the linearcombination will correspond to a transmission path whose quality isuseable. This is particularly true when the first weighting vectors arethe eigen vectors of the covariance matrix since, with these firstweighting vectors, the probabilities of destructive interference arestatistically uncorrelated.

[0031] In order to achieve as good a signal-to-noise ratio as possibleduring such a transmission using a linear combination of eigen vectors,the coefficients of the linear combination for a first weighting vectormay be chosen to be greater the greater its eigen value.

[0032] If the downlink signal delay is identical on two transmissionpaths, the subscriber station is unable directly to keep these twotransmission paths for the signal received by it separate from oneanother. It is thus possible for these two components to be in antiphaseat the subscriber station location, and hence to cancel one another out.Such mutual cancelation can be reliably avoided by producing a number ofdownlink signals at the base station from a user data sequence intendedfor that subscriber station, which downlink signals each have adifferent space time block coding, and each of these downlink signals istransmitted weighted with a different current weighting vector. In thisway, each propagation path has a characteristic associated space timeblock coding, which makes it possible to distinguish between thecomponents of the different transmission paths in all circumstances.

[0033] Exemplary embodiments will be explained in the following textwith reference to the drawing, in which:

[0034]FIG. 1 shows a block diagram of a mobile radio network,

[0035]FIG. 2 shows a block diagram of the base station;

[0036]FIG. 3 shows a block diagram of the subscriber station, and

[0037]FIG. 4 shows a flowchart of the method according to a firstrefinement; and

[0038]FIG. 5 shows a flowchart of the method according to a secondrefinement.

[0039]FIG. 1 shows the structure of a radio communications system inwhich the method according to the invention can be used. This comprisesa large number of mobile switching centers MSC, which are networked withone another and provide the access to a landline network PSTN.Furthermore, these mobile switching centers MSC are connected to in eachcase at least one base station controller BSC. Each base stationcontroller BSC in turn allows a connection for at least one base stationBS. A base station BS such as this can set up a message linkto-subscriber stations MS via a radio interface. At least some of thebase stations BS are for this purpose equipped with antenna devices AEwhich have a number of antenna elements (A₁−A_(M))

[0040]FIG. 1 shows, by way of example, connections V1, V2, Vk fortransmitting user information and signaling information betweensubscriber stations MS1, MS2, MSk, MSn and a base station BS. Anoperation and maintenance center OMC provides control and maintenancefunctions for the mobile radio network, or for parts of it. Thefunctionality of this structure can be transferred to other radiocommunications systems in which the invention can be used, in particularfor subscriber access networks with wire-free subscriber access.

[0041]FIG. 2 shows, schematically, the design of a base station BS. Asignal production device SA assembles the transmission signal, which isintended for the subscriber station MSk, into radio blocks, andallocates this signal to a frequency channel TCH. Atransmitting/receiving device TX/RX receives the transmission signalSk(t) from the signal production device SA. The transmitting/receivingdevice TX/RX comprises a beamforming network, in which the transmissionsignal Sk(t) for the subscriber station MSk is logically linked withtransmission signals s₁(t), s₂(t), . . . which are intended for othersubscriber stations and to which the same transmission frequency isallocated. For each subscriber signal and each antenna element, thebeamforming network has a multiplier M, which multiplies thetransmission signal s_(k)(t) by a component w_(m) ^((k)) of a weightingvector w^((k)), which is allocated to the receiving subscriber stationMSk. The output signals from the multipliers M which are in each caseallocated to one antenna element A_(m), m=1, . . . , M are added by anadder AD_(m), m=1,2, . . . , M, are converted to analog form by adigital/analog converter DAC, are converted to the transmissionfrequency (RF) and are amplified in a power amplifier PA before theyreach the antenna element (A₁, . . . , A_(M)). A structure which isanalogous to the described beamforming network but is not shownspecifically in the figure is arranged between the antenna elements (A₁,A₂, . . . , A_(M)) and a digital signal processor DSP, in order to breakdown the received mixture of uplink signals into the components for theindividual subscriber stations, and to supply these separately to theDSP.

[0042] A memory device SE contains a set of weighting vectors w^((k,1)),w^((k,2)), . . . for each subscriber station MSk, from which theweighting vector w^((k)) which is used by the multipliers M is selected.

[0043]FIG. 3 shows, schematically, the design of a subscriber stationMSk for carrying out a first refinement of the method according to theinvention.

[0044] The subscriber station MSk has a single antenna A, which receivesthe downlink signal emitted from the base station BS. After beingconverted to baseband, the received signal is supplied from the antennaA to a so-called rake searcher RS, which is used to measure delay timedifferences between the components of the downlink signal, which havereached the antenna A on different propagation paths. The receivedsignal is furthermore applied to a rake amplifier RA, which has a numberof rake fingers, three of which are illustrated in the figure, and eachof which has a delay element DEL and a despreader/descrambler EE. Thedelay elements DEL in each case delay the received signal by a delayvalue τ₁, τ₂, τ₃, . . . , which is supplied by the rake searcher RS. Thedespreaders/descramblers EE each produce a sequence of estimated symbolsat their outputs, and the results of the estimation process for theindividual descramblers may be different owing to the different phaseangles of the downlink signal with respect to the descrambling andspreading code in the individual fingers of the rake amplifier.

[0045] The symbol sequences produced by the despreaders/descramblers EEalso include the results of the estimation of training sequences whichare emitted by the base station, and which are characteristic andquasi-orthogonal for each antenna element of the base station. A signalprocessor SP is used to compare the results of the estimation of thesetraining sequences with the symbols which are known to the subscriberstation and are actually contained in the training sequences. Thiscomparison can be used to determine the impulse response of thetransmission channel between the base station BS and the subscriberstation MSk for each individual finger or tap. The outputs of thedespreaders/descrambler EE are also connected to a maximum ratiocombiner MRC, which assembles the individual estimated symbol sequencesto form a combined symbol sequence with the best-possiblesignal-to-noise ratio, which it supplies to a speech signal processingunit SSV. The method of operation of this unit SSV, which converts thereceived symbol sequence into a signal which is audible by a user andconverts received tones to a transmission symbol sequence, is well knownand does not need to be described here.

[0046] The signal processor SP determines the impulse responses of eachantenna element AE₁, . . . , AE_(M) individually for each tap, andcombines these impulse responses in the manner which is known, forexample, from the cited DE 198 03 188 to form a spatial covariancematrix, R_(xx). These spatial covariance matrices are passed to acomputation unit RE, whose method of operation will be described withreference to the flowchart shown in FIG. 4.

[0047] In an initialization phase 1, the computation unit RE adds alarge number of supplied covariance matrices separately for each tap andforms a mean value of the covariance matrices. This is followed by ananalysis of the eigen values and eigen vectors of the averagedcovariance matrices obtained for the various taps (step 2).

[0048] The analysis may extend into all the eigen vectors and values ofthe covariance matrix and, in the case under consideration here, acontrol unit KE determines a limited number, for example 2 or 4, of theeigen vectors found in the analysis process which have eigen values withthe largest magnitudes and which in consequence correspond to thetransmission paths with the least attenuation. Alternatively, it ispossible to use a method for eigen vector analysis which produces theeigen vectors of the covariance matrix in the sequence of decreasingmagnitudes of the eigen values, and which is terminated once the limitednumber of eigen values have been determined.

[0049] The coefficients of the determined eigen vectors w^((k,1)),w^((k,2)), . . . are combined with the user datastream coming from thespeech processing unit SSV and are transmitted via the antenna A to thebase station (step 4). The base station stores them in its memory unitSE for use as coefficients for the multipliers M for the beamformingnetwork.

[0050] The computation unit RE now switches to a working phase, in whichit in each case receives these covariance matrices R_(xx) from thesignal processor SP in each case related to an individual time slot ofthe subscriber station (step 5) and multiplies them by each of the eigenvectors which are stored in the memory unit and are transmitted to thebase station, in order to determine the eigen values of these vectorsfor the relevant covariance matrix (step 6). The number of the eigenvector which has the greater eigen value is transmitted to the basestation via the control unit KE in step 7. This eigen vector isidentified as dominant eigen vector since it makes the greatestcontribution, and generally the best contribution, to the receivedsignal. If only two determined eigen vectors are stored in the memoryelement SE and have been transmitted to the base station, one bit issufficient to identify the eigen vector with the respectively greatereigen value. In consequence, if one bit is available per time slot forfeeding back the reception characteristics to the base station, thevector which is used for beamforming by the base station can be updatedin each time slot and can be used for beamforming in the next time slot.

[0051] If four eigen values have been transmitted to the base station,two bits are required to identify the respective dominant eigen vector.If one bit is available per time slot for transmitting the receptioncharacteristics back, two time slots are thus required in order totransmit the complete identification of the dominant vector. Inconsequence, this is used for beamforming for the two time slotsfollowing its transmission; the identification to be used subsequentlyis transmitted in the course of these two slots.

[0052] The working phase steps may be repeated cyclically many timesbefore the initialization phase need be carried out once again, in orderto update the coefficients of the eigen vectors.

[0053] To assist understanding, a distinction has been drawn abovebetween the initialization phase and the working phase. However, thisdoes not mean that the two phases have to be carried out at separatetimes from one another. For example, it is possible and expedient tobracket the two phases with one another in that the computation unit REwith a received covariance matrix R_(xx) firstly determines the eigenvalues in step 6, and secondly uses this matrix to form a running meanvalue of the covariance matrixes in step 1. This ensures that anup-to-date averaged covariance matrix is available at all times, and canbe used to carry out the eigen value analysis in step 2.

[0054] A second refinement of the method according to the invention willbe described with reference to FIG. 5. In this refinement, the firstweighting vectors are determined on the basis of measurements of theuplink transmission from a subscriber station MSk to the base stationBS. For this purpose, the base station BS is equipped with componentsanalogous to the rake searcher RS, rake amplifier RA, signal processorSP, computation unit RE, memory element SE etc., described withreference to FIG. 3 for the subscriber station.

[0055] In step 1 of the method, the computation unit RE forms anaveraged covariance matrix for each individual tap of the uplink signal,and determines the eigen vectors and eigen values of the covariancematrix obtained in this way. These eigen values in each case correspondto one transmission path and contain the information relating to therelative phase angles of the corresponding magnitude of the uplinksignal to the individual antenna elements, and hence relating to thedirection from which that component is received. If the frequencies ofthe uplink and downlink in the radio communications system underconsideration are the same, the phase information contained in the eigenvector can be used directly for weighting the downlink signal. If thefrequencies of the uplink and downlink are different, then it isnecessary to convert the phase information contained in the eigen vectoron the basis of the uplink frequency to an appropriate direction, and inturn to convert this direction on the basis of the downlink frequency tophase information, in order to obtain eigen vectors which are suitablefor beamforming in the downlink.

[0056] The analysis in step 2 also includes the determination of theeigen values of the eigen vectors. The magnitude of the eigen vector isa measure of the quality of each individual transmission path; a givennumber of, for example, 2 or 4 eigen vectors are thus chosen forsubsequent use and are stored in step 3, these being the eigen vectorswhose eigen values have the highest magnitude of the eigen vectors whichhave been found.

[0057] In the subsequent working phase, the computation unit cyclicallyreceives covariance matrices from the signal processor, with eachcovariance matrix being related to in each case one individual tap ofthe uplink signal. The eigen vectors which are stored in the memory unitSE themselves each correspond to one specific tap. In step 6, thecomputation unit determines the current eigen value for each storedeigen vector by multiplying it by the covariance matrix which isproduced in step 5 and corresponds to the same tap as the eigen vector.The eigen value that is obtained provides a measure of the transmissionquality on the transmission path which corresponds to that eigen vector,with a time resolution which corresponds to the rate of production ofthe covariance matrices in the working phase. In this phase, thecovariance matrices are produced in real time in each case by the signalprocessor for each time slot allocated to that subscriber station; theeigen value is thus a measure of the transmission quality of thetransmission path, taking account of fast fading.

[0058] In a first simple variant of the method, this is followed by astep 8 in which a current weighting vector w^((k)) is calculated byforming a linear combination of the stored eigen vectors w^((k,1)),w^((k,2)), . . . , with each of the eigen vectors w^((k,1)), w^((k,2)),. . . being included in the linear combination multiplied by its eigenvalue or its magnitude obtained in step 6. The linear combination can benormalized. This weighting in the formation of the linear combinationensures that those transmission paths which have the best transmissioncharacteristics in the short term dominate the downlink signal which isemitted by the base station. The other eigen vectors which are includedin the current weighting vector w^((k)) are used to ensure that auseable signal arrives at the subscriber station even in a situationwhere the most highly weighted transmission path changes from one timeslot to the next.

[0059] If one of the transmission paths between the base station and thesubscriber station is a direct link, then this can be identified for thebase station by the fact that the corresponding component of thereceived uplink signal has relatively little phase fluctuation and,generally, little attenuation either. If such a direct transmission pathexists, the associated eigen vector can be used directly as the currentweighting vector w^((k)), in other words all the other eigen vectors areincluded in the formation of the linear combination with coefficients of0.

[0060] A further-developed variant of the second refinement ispredicated on a base station having an antenna device comprising anumber of antenna elements, which is able to transmit using space timebock codes. Codes such as these are known, for example from Tarokh etal., SpaceTime Block Codes from Orthogonal Designs, IEEE Trans. onInformation Theory, Volume 45 No. 5, July 1999. A detail of thetransmitting/receiving device Tx/Rx from such a base station is shown inFIG. 6. In this transmitting/receiving device, a complex-value symbolsequence which is intended for the subscriber station MSk is subdividedinto two branches, one of which contains a Space Time Block EncoderSTBE, which in this case reverses the sequence, conjugates and reversesthe mathematical sign of one symbol of two successive symbols in thesymbol sequence s_(k)(t). The two different symbol sequences obtained inthis way but having the same information content are weighted in abeamforming network, whose design is described analogously to that withreference to FIG. 2 and will therefore not be dealt with in any moredetail here, with two different eigen vectors w^((k,a)), w^((k,b)) fromthe set of eigen vectors w^((k,1)), w^((k,2)), . . . (w^((k,a))=(w₁^((k,a)), w₂ ^((k,a)), . . . , w_(m) ^((k,a))), which are additivelysuperimposed and are transmitted. The individual antenna elements (A₁, .. . A_(M)) are thus able to transmit a mixture of signals which havedifferent space time block coding. The coding is thus not specific foran individual antenna element but for a propagation path a or b, whichcorresponds to the respective eigen vector w^((k,a)) or w^((k,b)) usedfor weighting. This ensures that signals which reach the subscriberstation MSk on these two different transmission paths a, b can neverinterfere destructively even if their relative delay disappears. In thevariant of the second refinement of the method which uses thistransmitting/receiving device, the step 8 of formation of a linearcombination is thus replaced by the space time block coding. Apart fromthis, the method steps correspond; in particular, both variants have thecapability to interchange those of the stored eigen vectors which areincluded in the linear combination or are used for weighting the spacetime block-coded signals, from one cycle of the working phase to thenext.

[0061] Modifications of the refinements described here based on thedisclosure provided here are within the knowledge of those skilled inthe art. In particular, a variant is conceivable in which the eigenvectors are determined on the uplink signal, as described with referenceto the second refinement, and in which the determined eigen values aretransmitted from the base station to the subscriber station, so that thesubscriber station can carry out the method steps 5 to 7, as describedwith reference to FIG. 4 for the first refinement of the method.

1. A method for beamforming in a radio communications system havingsubscriber stations (MSk, MS1 to MSn) and having a base station (BS)which has an antenna device (AE) with a number of antenna elements (A₁to A_(M)), which emit a downlink signal in each case weighted withcoefficients w_(i), i=1, . . . M of a current weighting vector wcharacterized in that a) a number of first weighting vectors w^((j)) aredetermined in an initialization phase, and b) the current weightingvector w which is used for the emission of one time slot of the downlinksignal which is intended for the subscriber station (MSk) is cyclicallyredefined in a working phase on the basis of the determined firstweighting vectors.
 2. The method as claimed in claim 1, characterized inthat the first weighting vectors are determined on the basis ofmeasurements of the downlink transmission.
 3. The method as claimed inclaim 1 or 2, characterized in that a) the first weighting vectors w(i)are determined in the subscriber station, and the determined firstweighting vectors are transmitted to the base station, in theinitialization phase; and in that b) the subscriber station uses thedetermined first weighting vectors to select a dominant weightingvector, and transmits an identification of the dominant weighting vectorto the base station, in the operating phase.
 4. The method as claimed inclaim 3, characterized in that a first spatial covariance matrix of thereceived downlink signal is produced in the initialization phase, inthat eigen vectors of the first covariance matrix are determined, and inthat the eigen vectors are transmitted as the first weighting vectors.5. The method as claimed in claim 4, characterized in that the firstcovariance matrix is produced individually for each tap of the downlinksignal.
 6. The method as claimed in claim 4 or 5, characterized in thatthe determined first eigen vectors are those from the totality of eigenvectors of the first covariance matrix or matrices which have thelargest eigen values.
 7. The method as claimed in claim 4, 5 or 6,characterized in that the first covariance matrix is averaged over alarge number of time slots in the downlink signal.
 8. The method asclaimed in one of claims 4 to 7, characterized in that a second spatialcovariance matrix is produced cyclically in the operating phase, and inthat the weighting vector which is chosen as the dominant weightingvector of the determined eigen vectors is that which has the largesteigen value with the second covariance matrix.
 9. The method as claimedin one of claims 3 to 8, characterized in that each antenna elementperiodically emits a training sequence which is orthogonal to thetraining sequences of the other antenna elements, and in that the firstweighting vectors are determined on the basis of the training sequencesreceived by the subscriber station.
 10. The method as claimed in one ofclaims 3 to 9, characterized in that the number of determined firstweighting vectors is two, and in that the designation of the dominantweighting vector is transmitted in each time slot allocated to thesubscriber station.
 11. The method as claimed in claim 10, characterizedin that the designation for beamforming is used in the time slotimmediately following its transmission.
 12. The method as claimed in oneof claims 3 to 18, characterized in that the number of determined firstweighting vectors is 2^(n), n=2, 3, . . . , and in that the designation,which comprises n bits, of the dominant weighting vector is transmittedin portions of a bits, a=1, . . . , n, in each time slot allocated tothe subscriber station.
 13. The method as claimed in claim 12,characterized in that the designation for beamforming is inserted in then/a time slots which immediate follow its transmission.
 14. The methodas claimed in claim 1 or 2, characterized in that the first weightingvectors are determined on the basis of measurements of the uplinktransmission.
 15. The method as claimed in claim 14, characterized inthat a first spatial covariance matrix of the received uplink signal isproduced in the initialization phase, in that eigen vectors of the firstcovariance matrix are determined, and in that the eigen vectors are usedas first weighting vectors.
 16. The method as claimed in claim 15,characterized in that the first covariance matrix is producedindividually for each tap of the uplink signal.
 17. The method asclaimed in claim 15 or 16, characterized in that the determined eigenvectors are those from the totality of eigen vectors of the firstcovariance matrix or matrices which have the largest eigen values. 18.The method as claimed in claim 15, 16, or 17, characterized in that thefirst covariance matrix is averaged over a large number of time slots inthe uplink signal.
 19. The method as claimed in one of claims 15 to 18,characterized in that a second spatial covariance matrix is producedcyclically in the operating phase, and in that the weighting vectorwhich is chosen as the dominant weighting vector of the determined eigenvectors is that which has the largest eigen value with the secondcovariance matrix.
 20. The method as claimed in one of claims 15 to 19,characterized in that each subscriber station periodically emits atraining sequence, and in that the first weighting vectors aredetermined on the basis of the training sequences received by the basestation.
 21. The method as claimed in one of claims 1, 2, 14 to 20,characterized in that the current weighting vector is a linearcombination of the first weighting vectors.
 22. The method as claimed inclaim 15 and claim 21, characterized in that the coefficients of thelinear combination for a first weighting vector are chosen to be greaterthe greater its eigen value.
 23. The method as claimed in one of claims1, 2, 14 to 20, characterized in that a number of downlink signals,which each have a different space time block coding, are produced from asymbol sequence which is intended for the subscriber station (MSk), andin that each of the downlink signals is emitted weighted with adifferent current weighting vector.
 24. The method as claimed in claim21 or 22, characterized in that the current weighting vector is chosenfrom the first weighting vectors when an LOS transmission path existsbetween the base station and the subscriber station.